Ammonium sulphate is the recommended fertilizer for pecan trees until the tree begins producing nuts. For a mature, nut-producing tree, a 10-10-10 blend of fertilizer is recommended, with 10 percent each of nitrogen, phosphorus and potassium.
Ammonium sulphate decreases the pH of the soil and increases the zinc, magnesium and iron available to the tree. Beginning with the tree's second year of growth, 1/4 to 1/2 pound of ammonium sulphate should be applied every six weeks until the month of June. Applying fertilizer after June causes the tree to retain its leaves too long into the fall, risking winter injury. After the third year, 2 pounds of ammonium sulphate should be applied twice a year. For a nut-bearing tree, 4 pounds of 10-10-10 fertilizer should be applied for each inch of trunk diameter, measured below the scaffold branches. Fertilizer should never be applied closer than 12 inches to the trunk.